Various methods of non-destructive testing and/or monitoring of structures are known. One such method is acoustic emission testing, which detects acoustic emissions (i.e. stress waves) generated in a material when discontinuity growth occurs in same. Discontinuity growth results from fatigue, plastic deformation, cracking, brittle fracture, corrosion pitting and the like. The acoustic emissions or stress waves which are of interest for non-destructive testing purposes take the form of low amplitude pulses, in the 0.1 to 2 MHz frequency range. Acoustic emission testing is a useful means of detecting impending or incipient failure of a structure, since such testing can detect discontinuity growth before it is visible. Acoustic emission equipment can also detect the existence of leaks, in the case of structures containing gases or fluids.
Conventional acoustic emission testing equipment generally consists of one or more piezoelectric transducers which are attached to the surface of the structure being tested and which are coupled to a data analysis unit of one or more channels. The output of each transducer is typically amplified, conditioned (such as by filtering) and then analyzed by the data analysis unit. Typical parameters generated by the analysis unit include emission counts, count rate, amplitude and energy. These parameters are generally displayed as a function of time on either a hard copy recorder or a video display terminal, and are reviewed and interpreted by a trained operator, to determine the existence and nature of any discontinuities or leaks.
Most acoustic emission testing systems require that a source of stress, such as hydrostatic pressure, be applied thereto, in order to determine the existence of discontinuity growth. This type of equipment, which is geared to periodic and proof testing of structures, is limited, since it cannot provide any monitoring of the structure during operating conditions, i.e. it cannot provide on-line monitoring capability.
Some acoustic emission testing equipment is said to be capable of providing limited on-line monitoring to detect certain discontinuities, for some applications. For instance, U.S. Pat. No. 4,380,172, which issued to Imam et al on Apr. 19, 1983, discloses a method for detecting incipient cracks in the rotor of a fluid powered turbine; and U.S. Pat. No. 4,317,368, which issued to McElroy on Mar. 2, 1982, discloses an apparatus which detects acoustic emissions produced in a fibreglass boom by breakage of glass fibres.
However, these and other known acoustic emission testing or monitoring systems are not well adapted to economically analyze the output of more than only a few acoustic emission detectors, since multichannel analyzers of more than only a few channels are expensive. Most of these systems are also very expensive to operate continuously for more than a few hours of time, since generally a highly trained scientist or technician must be continuously present to interpret the output of the analyzer and make decisions based thereon. In particular, it has been found that conventional acoustic emission monitoring equipment is incapable of economically monitoring a large structure such as a pipeline extending for several kilometers under operating conditions on a continuous, long-term basis (i.e. 24 hours a day for days or weeks), in view of the large number of detectors required (several hundred for some cases), and in view of the overwhelming amount of data which is generated therefrom and which must be analyzed and interpreted, in order to obtain meaningful results.